This invention relates to a base plate for optical discs capable of storing, recording and reproducing audio and video information.
The optical disc base plate used for digital audio discs, video discs, optical disc recording media, etc., is an approximately 1 mm thick transparent plate carrying an information pattern formed by grooves and/or pits on the surface.
The following three methods have been typically known for forming these optical disc base plates.
(1) A melt of a high polymeric material such as polycarbonate, polymethyl methacrylate and the like is injected into a mold incorporated with a metal-made stamper having an information pattern of grooves and/or pit, and then said melt in the mold is cooled and molded into a transparent plate carrying the information pattern [see Nikkei Mechanical, page 34 (Feb. 1, 1982), and Nikkei Electronics, page 133 (June 7, 1982)].
(2) A metal-made stamper formed with an information pattern is placed on a plastic or glass-made transparent substrate with a thin layer of a liquid light-curable resin interposed therebetween, then this assembly is irradiated via the transparent substrate with light to cure said light-curable resin, and thereafter the stamper is separated from the layer of said cured resin which is kept adhering to said transparent substrate to thereby obtain a transparent plate having the information pattern (see Japanese Patent Kokai (Laid-Open) Nos. 86756/78 and 152028/80).
(3) A stamper bearing an information pattern and a light-transmittable flat plate are placed parallel to each other to form a space therebetween and a light-curable resin is then cast into said space and cured via said light-transmittable plate by irradiation of light, after which said stamper and light-transmittable plate are removed to obtain a transparent plate of cured resin carrying the information pattern (see Japanese Patent Kokai (Laid-Open) No. 160338/80).
These known methods, however, had the following problems.
(a) The optical disc base plate obtained according to the method (1), although excellent in productivity and workability, is unable to perfectly eliminate the molecular orientation which occurs when the high polymeric material is fluidized and solidified, and consequently optical anisotropy occurs in the base plate to cause noise when the information signal is read or written. There is a tendency that such optical anisotropy in the base plate becomes conspicuous especially when attempting to obtain a heat-resistant base plate by raising the thermal deformation temperature of the plate material, and the obtained plate proves optically defective for practical use. The optical disc base plate obtained by the method (1) also had difficulty in transferring the information pattern faithfully from the stamper to the base plate.
(b) The optical disc base plate obtained from the method (2) has good information pattern transfer characteristics, but since a transparent substrate must be prepared separately, the number of the steps in its production process increases, which results in increasing manufacturing cost. Also, it is difficult to obtain strong adhesion between the transparent substrate and the light-curable resin thereby making it difficult to maintain high reliability for a long period of time under high-temperature, high-humidity conditions.
(c) The optical disc base plate obtained by the method (3), although excellent in workability, etc., involves difficulties in the selection of appropriate light-curable resin and molding conditions, and it is difficult with this method to obtain a base plate having excellent mechanical strength, heat resistance and moisture resistance with minimum optical anisotropy.
According to the method (3) which is most commonly used, a transparent plate made of glass or like material and a stamper having an information pattern are placed parallel and opposed to each other so as to form a space therebetween through an outer peripheral ring secured to the outer periphery of said stamper, then a light-curable resin is cast into said space from an inlet formed at a central part of said stamper, and said light-curable resin is then cured via the transparent plate by irradiation of energy beams from a light source.
The generally used acrylate- or methacrylatetype light-curable resins and polyene- or polythiol-type light-curable resins shrink and are reduced in volume when cured, so that the cured product has sinks or voids in its outer peripheral portion.
In order to solve this problems, various proposals have been made, such as arranging the stamper to be movable (Japanese Pat. Laid-Open No. 160338/80) or using an elastic ring (Japanese Pat. Laid-Open No. 25921/82) to suppress the shrinkage of the cured product.
These methods, however, have their own problems such that the base plate made of the cured product of light-curable resin becomes non-uniform in thickness, making it difficult to control the external dimensions of the base plate, and a slight amount of light-curable resin which has gotten between the outer peripheral ring and the stamper or between said ring and the transparent substrate is improperly cured due to oxygen in the air to cause contamination of the base plate by the half-cured resin or deposition of the half-cured adherent residual resin to adversely affect the dimensional precision of the mold.